In an OFDMA communication system, the time-frequency resources of the system are shared among a plurality of mobile stations. A base station assigns resources to mobile stations using an assignment message, which is transmitted as part of a control channel. To minimize control channel overhead, it is known for the base station to utilize a channel tree, wherein the channel tree is comprised of nodes, wherein each channel tree node corresponds to a specific portion of the time-frequency resources of the system.
In some OFDMA communication systems, the time-frequency resources are divided into multiple regions (also called zones) to facilitate different types of transmissions. For example, for time division duplex (TDD) systems, the time domain is divided into a downlink (DL) region and an uplink (UL) region. In some systems, the DL region and UL region are further divided into additional regions. For example, the DL may be divided into a partial usage of subcarriers (PUSC) region and a full usage of subcarriers (FUSC) region such as described by the IEEE 802.16 standard. Mobile stations assigned to the DL PUSC region experience less interference than mobile stations assigned to the DL FUSC region. Therefore, the DL PUSC region is often advantageous for mobile stations near the cell edge. The DL FUSC region utilizes the entire bandwidth in each sector, thereby maximizing the spectral efficiency. The DL FUSC region is advantageous for those mobile stations that can tolerate increased interference relative to what would be seen in the DL PUSC region and is therefore advantageous for mobile stations near the base station.
Within each region, subchannels are defined, wherein each subchannel is a subset of the entire frequency resources. The subchannel definition may be different for the different regions. In addition, the minimum time-frequency assignment in each region may be different.
It is known to define a channel tree when there is only one region. However, it is not known how to create a channel tree for systems that allow multiple regions with dynamic boundaries. Thus, there is a need for creating a channel tree, for the case when there are multiple dynamic regions, for efficiently allocating time-frequency resources.